The present invention is an improvement in combustion chambers and particularly an improvement of the device described in U.S. Pat. No. 4,085,581, to which reference will be made herein. The improvement is intended for combustion installations, particularly gas turbine engines for turbojet aircraft. The installations to which the invention applies comprise a combustion chamber defined by annular walls traversed by lateral openings for the transverse penetration of air, and an end wall in the shape of a dome.
The injection of the fuel is effected by prevaporizing pipes secured to the end of the chamber, said pipes protruding into the chamber proper and consisting of a hollow structure comprising an inlet portion connected with sources of liquid fuel and or air, and of at least one outlet portion terminating in an escape orifice facing the dome constituting the end of the combustion chamber.
In order to protect the end of the combustion chamber from the impact of the hot jet coming from the prevaporizing pipes, it was found to be advantageous to interpose in the trajectory of the hot jets shield cups which play the role of heat shields and which have no other structural function. In the case of annular chambers it is of advantage to provide each pipe of prevaporization with such a cup having the configuration of a section of a crown, the entirety of the cups, once installed, forming an essentially continuous crown defining an annular space together with the end of the combustion chamber or dome.
Because the cups are exposed to high thermal stresses particularly in the zones of the impact of the jets issuing from the prevaporizing pipes, attempts were made to cool said cups by introducing air through the dome, into the annular space defined between the cups and the dome. Various solutions have been proposed to improve the efficiency of the cooling process, such as that described in French Pat. No. 2,152,465. In a general manner, in the first known embodiments, the cooling air was introduced through orifices of the dome and circulated behind the cup and then evacuated around its periphery toward the combustion zone of the chamber.
This mode of realization has a certain number of disadvantages; in particular, at low ratings a portion of the fuel trickles down the cold wall presented by the cup without participating in the combustion process, to the extent to which this portion subsequently reaches the walls of the chamber itself and is entrained by the cooling films of the chamber. In addition, the combustion reactions rapidly becoming limited in the proximity of the cold wall. This results in non-negligible rates of pollutant emission at these speeds, particularly of unburned hydrocarbons and CO.
The U.S. patent referred to proposed to reduce the emission of pollutants and thus to improve the yield of such combustion chambers, by piercing the cups with a large number of small orifices in order to generate high velocity air jets which on the one hand atomize the fuel trickling onto the cups and on the other hand increase the local turbulence. These orifices are supplied with air from a supply chamber formed in the space between the dome and the cups by means of small walls. The small walls are carried on the face of the cup turned toward the dome, are supported by said dome and divide the space between the dome and the cups, on the one hand, into a lateral annular double region where cooling takes place successively by means of the impact of the ends of the cup and by the film of air at the head of the chamber, and on the other hand, into a central annular region constituting a supply chamber, where the cup has orifices inclined toward the outside of the combustion chamber and dimensioned so as to create a plurality of air jets at high velocities.
These arrangements resulted in an appreciable improvement of combustion yields at reduced ratings for the type of chamber described, as indicated hereinabove.